Honeycomb structural body

ABSTRACT

An object of the present invention is to provide a honeycomb structural body with a long service life, which can reduce a pressure loss to a low level upon collecting particulates and maintain the pressure loss at the low level for a long time even after regenerating processes. The honeycomb structural body includes a columnar porous ceramic block in which a large number of through holes are placed in parallel with one another in the length direction with a wall portion interposed therebetween. Herein, the large number of through holes are constituted by a group of large-capacity through holes, each of which is sealed at one end of the honeycomb structural body so that the total sum of the areas on a cross section perpendicular to the length direction is made relatively great, and a group of small-capacity through holes, each of which is sealed at the other end of the honeycomb structural body so that the total sum of the areas on the cross section is made relatively small, and a surface roughness R y  of the wall face of the through hole is set in a range from 10 to 100 μm.

TECHNICAL FIELD

This application claims benefit of priority to Japanese PatentApplication No. 2003-161261, filed on Jun. 5, 2003, the contents ofwhich are incorporated by reference herein. The present inventionrelates to a honeycomb structural body that is used as a filter forremoving particulates and the like contained in exhaust gases dischargedfrom an internal combustion engine such as a diesel engine or the like.

BACKGROUND ART

In recent years, particulates such as soot contained in exhaust gasesdischarged from internal combustion engines of vehicles such as buses,trucks and the like and construction machines have raised seriousproblems as those particulates are harmful to the environment and thehuman body.

There have been proposed various ceramic filters which allow exhaustgases to pass through porous ceramics and collect particulates in theexhaust gases, thereby purifying the exhaust gases.

Conventionally, with respect to the honeycomb filter of this type, afilter having the following structure has been proposed in which: twokinds of through holes, that is, a through hole with a relatively largercapacity (hereinafter, referred to as large-capacity through hole) and athrough hole with a relatively smaller capacity (hereinafter, referredto as small-capacity through hole) are prepared, and the end on theexhaust gas outlet side of the large-capacity through hole is sealedwith a plug, with the end on the exhaust gas inlet side of thesmall-capacity through hole being sealed with a plug, so that thesurface area of the through hole with the opened inlet side(hereinafter, referred to as inlet-side through hole) is made relativelygreater than the surface area of the through hole with the opened outletside (hereinafter, referred to as outlet-side through hole); thus, itbecomes possible to suppress an increase in pressure loss uponcollecting particulates (for example, see Patent literature 1, and FIG.17 of Patent literature 2).

Moreover, another filter has been disclosed in which: the number of theinlet-side through holes is made greater than the number of theoutlet-side through holes, so that the surface area of the inlet-sidethrough holes is made relatively greater than the surface area of theoutlet-side through holes; thus, it becomes possible to suppress anincrease in a pressure loss upon collecting particulates (for example,see FIG. 3 of Patent literature 2).

In the case of the honeycomb filter used in filters for purifyingexhaust gases, disclosed in Patent literature 1 and Patent literature 2,in comparison with a honeycomb filter in which the total amount of thesurface area of the inlet-side through holes and the total amount of thesurface area of the outlet-side through holes are the same, since thesurface area of the inlet-side through holes is relatively greater, withthe result that the deposition layer of collected particulates becomesthinner, thereby making it possible to suppress an increase in apressure loss at the time of collecting particulates.

Moreover, after having collected a predetermined amount of particulates,an engine controlling process is carried out through a post injectionsystem or the like to raise the exhaust gas temperature and thetemperature of a heater placed on the upstream side of exhaust gasesfrom the honeycomb structural body is raised so that, upon burningparticulates, the particulates are made in contact with high-temperaturegases to be easily burned, making it possible to accelerate the burningspeed of the particulates.

However, in the above-mentioned conventional honeycomb filters, ashesthat remain as dregs after particulates have been burnt are accumulatedon the wall face of the through holes as they are without being moved.For this reason, the problems with the above-mentioned structures arethat pores, formed in the partition wall, are closed and that the ashestend to form bridges to cause clogging in the through holes, resultingin an abrupt rise in the pressure loss.

Moreover, in the case of the honeycomb filter shown in FIG. 17 of Patentliterature 2, as the surface area of the large-capacity through holes ismade relatively greater, the weight of the honeycomb structural bodyconstituting the honeycomb filter tends to decrease, resulting in areduction in the thermal capacity and the subsequent good thermalresponse. Consequently, the burning speed of particulates becomes toofast, with the result that ashes are deposited on the wall faces of thethrough holes, as they are, without being moved, and the ashes tend toform bridges to cause clogging in the through holes, resulting in anabrupt rise in the pressure loss.

-   Patent literature 1: Patent gazette No: 3130587-   Patent literature 2: U.S. Pat. No. 4,417,908 (FIG. 3, FIG. 17 and    the like)

DISCLOSURE OF THE INVENTION The Problem that the Invention is to Solve

The present invention has been devised so as to solve theabove-mentioned problems, and it is an object thereof to provide ahoneycomb structural body (filter) with a long service life, which canreduce a pressure loss to a low level upon collecting particulates andmaintain the pressure loss at the low level for a long time even afterregenerating processes.

The Means for Solving the Problem

The present invention is a honeycomb structural body made of a columnarporous ceramic block in which a large number of through holes are placedin parallel with one another in the length direction with a wall portioninterposed therebetween,

-   -   wherein    -   said large number of through holes comprises:    -   a group of large-capacity through holes, each of which is sealed        at one end of said honeycomb structural body such that the total        sum of the areas thereof on a cross section perpendicular to the        length direction is made relatively great; and    -   a group of small-capacity through holes, each of which is sealed        at the other end of said honeycomb structural body such that the        total sum of the areas on said cross section is made relatively        small,    -   a surface roughness R_(y) of the wall face of said through hole        being set in a range from 10 to 100 μm.

The following description will discuss the honeycomb structural body ofthe present invention.

In the present invention, the surface roughness (greatest height) R_(y)of the wall face of the through hole, measured based upon JIS B 0601, isset in a range from 10 to 100 μm; therefore, the pores and grains on thethrough hole wall face are properly placed to form appropriateirregularities so that the deposition state of particulates is allowedto change depending on these irregularities, making it possible toreduce a pressure loss to a low level upon collecting particulates.

Moreover, upon carrying out a regenerating process, the resulting ashesare easily moved through the through holes to the outlet side of exhaustgases, making it possible to reduce clogging caused by ashes depositedon the wall face of the through hole; therefore, it becomes possible toeffectively utilize the capacity of the large-capacity through hole, tomaintain a pressure loss in a low level for a long time, to reduce loadimposed on the engine, and consequently to provide a honeycombstructural body having a long service life. Thus, it becomes possible tocut maintenance costs required for back washing and the like.

In the present invention, the mechanism that makes the pressure losslower has not been sufficiently clarified; however, the mechanism ispresumably explained as follows:

The honeycomb structural body of the present invention has the group oflarge-capacity through holes and the group of small-capacity throughholes, and the aperture rates of the two end faces are different fromeach other. The honeycomb structural body of this type has such astructure that a proportion of the partition wall located between thethrough holes constituting the group of large-capacity through holesbecomes greater. In other words, a proportion of the partition walllocated between the through holes constituting the group oflarge-capacity through holes and the through holes constituting thegroup of small-capacity through holes becomes smaller.

Therefore, this structure makes it difficult for gases to directly flowfrom the through holes constituting the group of large-capacity throughholes to the through holes constituting the group of small-capacitythrough holes. For this reason, in comparison with the honeycombstructural body in which the aperture rates of the two end faces are thesame, the flow rate of gases flowing into the partition wall becomesgreater, in the case of the same displacement of engines, with theresult that high-density particulates and ashes are formed and easilyallowed to penetrate the through holes deeply.

In addition to the above-mentioned structure, the honeycomb structuralbody of the present invention is designed to have a predeterminedsurface roughness on its through-hole wall face. When the surfaceroughness of the through-hole wall face is made higher to a certaindegree, the deposition state of soot and ashes at the correspondingportion becomes irregular and the gas flow is locally changed so that itis possible to prevent too much soot and ashes from entering the walland also to easily exfoliate the soot and ashes; thus, it becomespossible to avoid forming a thick deposition layer, and consequently toreduce the pressure loss.

Therefore, in the honeycomb structural body of the present invention,although the amount of deposition of soot and ashes partially increases,the soot and ashes are easily exfoliated, with the result that thepressure loss becomes smaller.

In the case when the surface roughness (greatest height) R_(y) of thewall face, measured based upon JIS B 0601, exceeds 100 μm, extremelyhigh local portions and extremely low local portions are present on thepartition wall. Further, in the case when the surface roughness is toolarge, since particulates are deposited on the wall face of the throughhole irregularly, or are deposited in a manner so as to invade into thewall, some portions having remaining ashes and other portions having noremaining ashes are formed on the wall face and inside the wall, and itis assumed: those portions having more remaining ashes are apt to haveclogging and bridge formation, resulting in a high pressure loss.

In the case when the surface roughness (greatest height) R_(y) of thewall face, is less than 10 μm, it is considered that the wall face ofthe through hole becomes flat, and the flat wall face makes it difficultfor gases to flow therein, failing to provide the above-mentionedexfoliating effect to cause a high pressure loss. Moreover, in the casewhen particulates form a deposition layer on the wall face with beingdeposited on ashes with a high density, the ashes are condensed (with anincreased bulk density) and become difficult to be exfoliated. When itbecomes difficult for gases to flow therein, it becomes difficult toburn the soot, resulting in a difficulty in carrying out a regeneratingprocess and the subsequent increase in the pressure loss. Moreover,since the honeycomb structural body becomes close to a compact state,even a small amount of particulate deposition causes an abrupt increasein the pressure loss, resulting in a great load on the engine and thesubsequent instability in the amount of discharged particulates.Consequently, the collecting state of particulates becomes irregular,and upon regenerating, ashes tend to form bridges, with the result thatclogging in the pores tends to occur to also cause an increase in thepressure loss.

Here, the surface roughness (greatest height) R_(y) of the wall facemeasured based upon JIS B 0601 refers to a value obtained through thefollowing processes: a standard length is drawn from a roughness curvein the direction of its average line, and with respect to the drawnportion, the distance between the peak line and the bottom line ismeasured in the direction of longitudinal magnification of the roughnesscurve; thus, the resulting value is indicated by a unit of μm.

The Effect of the Present Invention

The honeycomb structural body of the present invention makes it possibleto suppress an increase in pressure loss upon collecting particulates.Moreover, the honeycomb structural body of the present invention alsomakes it possible to maintain the pressure loss caused by ash depositionat a low level for a long time even after regenerating processes, andconsequently to effectively utilize the capacity of the large-capacitythrough holes; thus, it becomes possible to reduce a load imposed on theengine, and to provide a honeycomb structural body having a long servicelife. Thus, it becomes possible to cut maintenance costs required forback washing and the like.

The Best Mode for Carrying Out the Present Invention

A honeycomb structural body of the present invention is a honeycombstructural body made of a columnar porous ceramic block in which a largenumber of through holes are placed in parallel with one another in thelength direction with a wall portion interposed therebetween,

-   -   wherein    -   said large number of through holes comprises:    -   a group of large-capacity through holes, each of which is sealed        at one end of said honeycomb structural body such that the total        sum of the areas thereof on a cross section perpendicular to the        length direction is made relatively great; and    -   a group of small-capacity through holes, each of which is sealed        at the other end of said honeycomb structural body such that the        total sum of the areas on said cross section is made relatively        small,    -   a surface roughness R_(y) of the wall face of said through holes        being set in a range from 10 to 100 μm.

The honeycomb structural body of the present invention is made of acolumnar porous ceramic block in which a large number of through holesare placed in parallel with one another in the length direction with awall portion interposed therebetween, however, the porous ceramic blockmay be constituted by combining a plurality of columnar porous ceramicmembers, each having a plurality of through holes that are placed inparallel with one another in the length direction with partition wallinterposed therebetween, with one another through sealing materiallayers (hereinafter, also referred to as an aggregated honeycombstructural body), or may be formed by ceramic members that areintegrally sintered as one unit as a whole (hereinafter, also referredto as an integral honeycomb structural body).

Moreover, the honeycomb structural body may contain the porous ceramicblock, with a sealing material layer being formed on the circumferencethereof.

In the case of the aggregated honeycomb structural body, the wallportion is constituted by a partition wall that separates through holesof porous ceramic members, an outer wall of the porous ceramic memberand a sealing material layer that serves as a bonding agent layerbetween the porous ceramic members, and in the case of the integralhoneycomb structural body, the wall portion is formed by a partitionwall of one kind.

Moreover, the large number of through holes formed in the honeycombstructural body may comprise: a group of large-capacity through holes,each of which is sealed at one end of the honeycomb structural body suchthat the total sum of the areas on a cross section perpendicular to thelength direction is made relatively great, and a group of small-capacitythrough holes each of which is sealed at the other end of the honeycombstructural body such that the total sum of the areas on theabove-mentioned cross section is made relatively small.

Here, each of the through holes may have the same area in the crosssection perpendicular to the length direction of the through holes, andthe number of the through holes constituting the group of large-capacitythrough holes with one end being sealed is made greater than the numberof the through holes constituting the group of small-capacity throughholes with the other end being sealed, or the area in the cross sectionperpendicular to the length direction of the through holes constitutingthe group of large-capacity through holes with one end being sealed maybe made relatively greater, while the area in the cross sectionperpendicular to the length direction of the through holes constitutingthe group of small-capacity through holes with the other end beingsealed is made relatively smaller.

Further, in the latter case, not particularly limited, as long as thetotal sum of the areas on a cross section perpendicular to the lengthdirection of the through holes constituting the group of large-capacitythrough holes is made greater than the total sum of the areas on thecross section perpendicular to the length direction of the through holesconstituting the group of small-capacity through holes, the number ofthrough holes constituting the group of large-capacity through holes andthe number of through holes constituting the group of small-capacitythrough holes may be the same or different from each other.

Moreover, in the honeycomb structural body of the present invention,shapes serving as basic units are repeated, and from the viewpoint ofthe basic units, the area ratios in the cross section are different fromeach other. Therefore, in the case when a specific structure is includedin the honeycomb structural body of the present invention whenmeasurements are strictly carried out up to one or two cells on thecircumference, the calculations need to be carried out by excluding theone or two cells, or the calculations need to be carried out except forportions that are not repetitions of the basic units so that adetermination is made as to whether or not the structure is included inthe present invention. More specifically, for example, as shown in FIG.8, a honeycomb structural body having any structure in which, in thecase when, with respect to the shape of a cross section perpendicular tothe length direction of the through holes, the cross-sectional shapesexcept for those in the vicinity of the circumference are the same,sealed portions and opened portion of each of the ends are placed in amanner so as to form a staggered pattern as a whole, is determined notto be included in the honeycomb structural body of the presentinvention.

FIG. 1 is a perspective view that schematically shows a specific exampleof an aggregated honeycomb structural body that is one example of thehoneycomb structural body of the present invention, FIG. 2(a) is aperspective view that schematically shows one example of a porousceramic member that forms the honeycomb structural body shown in FIG. 1,and FIG. 2(b) is a cross-sectional view taken along line A-A of theporous ceramic member shown in FIG. 2(a). In the honeycomb structuralbody shown in FIG. 1, the large number of through holes are constitutedby two kinds of through holes, that is, large-capacity through holeseach of which has a comparatively large area on the cross sectionperpendicular to the length direction and small-capacity through holeseach of which has a comparatively small area on the above-mentionedcross section.

As shown in FIG. 1, the honeycomb structural body 10 of the presentinvention has a structure in which a plurality of porous ceramic members20 are combined with one another through sealing material layers 14 toform a ceramic block 15, with a sealing material layer 13 used forpreventing exhaust-gas leak being formed on the periphery of thisceramic block 15. Here, the sealing material layer is formed, ifnecessary.

Here, in the porous ceramic member 20, a large number of through holes21 are placed in parallel with one another in the length direction, andthe through holes 21 are constituted by two kinds of through holes, thatis, large-capacity through holes 21 a each of which has a comparativelylarge area on the cross section perpendicular to the length directionand small-capacity through holes 21 b each of which has a comparativelysmall area on the above-mentioned cross section, and each of thelarge-capacity through holes 21 a is sealed with a plug 22 at the end onthe exhaust-gas outlet side of the honeycomb structural body 10, whileeach of the small-capacity through holes 21 b is sealed with a plug 22at the end on the exhaust-gas inlet side of the honeycomb structuralbody 10; thus, a partition wall 23, which separate these through holes,is allowed to function as filters. In other words, exhaust gases thathave entered the large-capacity through holes 21 a are allowed to flowout of the small-capacity through holes 21 b after necessarily passingthrough the partition wall 23.

In the honeycomb structural body 10 shown in FIG. 1, the shape is formedas a column shape; however, not particularly limited to the columnshape, for example, any desired shape such as an elliptical column shapeand a rectangular pillar shape may be used.

In the honeycomb structural body of the present invention, with respectto the material for the porous ceramic material, not particularlylimited, examples thereof include: nitride ceramics such as aluminumnitride, silicon nitride, boron nitride and titanium nitride; carbideceramics such as silicon carbide, zirconium carbide, titanium carbide,tantalum carbide and tungsten carbide; oxide ceramics such as alumina,zirconia, cordierite, mullite and the like. Moreover, the honeycombstructural body of the present invention may be made of a compositematerial of silicon and silicon carbide or the like, or may be made ofaluminum titanate. Among these, silicon carbide, which has high heatresistance, superior mechanical properties and high thermalconductivity, is desirably used.

Although not particularly limited, the porosity of the porous ceramicmember is preferably set in a range from 20 to 80%. When the porosity isless than 20%, the honeycomb structural body of the present invention ismore likely to cause clogging, while the porosity exceeding 80% causesdegradation in the strength of the porous ceramic member, with theresult that it might be easily broken. Since the wall face roughness ofthe through holes also varies depending on the porosity of the honeycombstructural body, the honeycomb structural body needs to be produced bytaking into consideration factors, such as macroscopic flatness in whichno pores are taken into consideration and porosity, so that the surfaceroughness (greatest height) R_(y) of the wall face, measured based uponJIS B 0601, is set in a range from 10 to 100 μm.

Here, the above-mentioned porosity can be measured through knownmethods, such as a mercury press-in method, Archimedes method and ameasuring method using a scanning electronic microscope (SEM).

The average pore diameter of the porous ceramic members is preferablyset in a range from 1 to 100 μm. The average pore diameter of less than1 μm tends to cause clogging of particulates easily. In contrast, theaverage pore diameter exceeding 100 μm tends to cause particulates topass through the pores, with the result that the particulates cannot becollected, making the members unable to function as a filter.

With respect to the particle size of ceramic particles to be used uponmanufacturing the porous ceramic members, although not particularlylimited, those which are less susceptible to shrinkage in the succeedingsintering process are preferably used, and for example, those particles,prepared by combining 100 parts by weight of particles having an averageparticle size from 0.3 to 50 μm with 5 to 65 parts by weight ofparticles having an average particle size from 0.1 to 1.0 μm, arepreferably used. By mixing ceramic powders having the above-mentionedrespective particle sizes at the above-mentioned blending ratio, it ispossible to provide a porous ceramic member.

Moreover, by adjusting the particle sizes of the above-mentioned twokinds of powders, in particular, the particle size of the powder havingthe greater particle size, the wall face roughness of the through holescan be adjusted. In the case when an integral honeycomb structural bodyis produced, the same method can be used.

The above-mentioned plug is preferably made of porous ceramics. In thehoneycomb structural body of the present invention, since the porousceramic member with one end sealed with the plug is made of porousceramics, by making the plug using the same porous ceramics as theporous ceramic member, it becomes possible to increase the bondingstrength between the two materials, and by adjusting the porosity of theplug in the same manner as that of the above-mentioned porous ceramicmember, it is possible to take the matching of the coefficient ofthermal expansion of the porous ceramic member and the coefficient ofthermal expansion of the plug; thus, it becomes possible to prevent theoccurrence of a gap between the plug and the partition wall due to athermal stress that is exerted upon production as well as upon use andthe occurrence of a crack in the plug or the portion of the partitionwall with which the plug comes in contact.

In the case when the plug is made from porous ceramics, with respect tothe material thereof, not particularly limited, the same material as theceramic material forming the porous ceramic member may be used.

In the honeycomb structural body of the present invention, the sealingmaterial layers 13 and 14 are formed between the porous ceramic members20 as well as on the periphery of the ceramic block 15. Further, thesealing material layer 14, formed between the porous ceramic members 20,also serves as a bonding agent that binds a plurality of porous ceramicmembers 20 with one another, and the sealing material layer 13, formedon the periphery of the ceramic block 15, serves as a sealing materialused for preventing leak of exhaust gases from the peripheral portion ofthe ceramic block 15, when the honeycomb structural body 10 of thepresent invention is placed in an exhaust passage of an internalcombustion engine.

With respect to the material for forming the sealing material layer, notparticularly limited, examples thereof include an inorganic binder, anorganic binder and inorganic fibers and/or inorganic particles. Here, asdescribed above, in the honeycomb structural body of the presentinvention, the sealing material layer is formed between the porousceramic members as well as on the periphery of the ceramic block; andthese sealing material layers may be made from the same material ormaterials different from each other. Moreover, in the case when thesealing material layers are made from the same material, the blendingratios of the materials may be the same or different from each other.

With respect to the inorganic binder, for example, silica sol, aluminasol and the like may be used. Each of these may be used alone or two ormore kinds of these may be used in combination. Among the inorganicbinders, silica sol is more preferably used.

With respect to the organic binder, examples thereof include polyvinylalcohol, methyl cellulose, ethyl cellulose and carboxymethyl cellulose.Each of these may be used alone or two or more kinds of these may beused in combination. Among the organic binders, carboxymethyl celluloseis more preferably used.

With respect to the inorganic fibers, examples thereof include ceramicfibers, such as silica-alumina, mullite, alumina and silica. Each ofthese may be used alone or two or more kinds of these may be used incombination. Among the inorganic fibers, silica-alumina fibers are morepreferably used.

With respect to the inorganic particles, examples thereof includecarbides, nitrides and the like, and specific examples include inorganicpowder or whiskers made from silicon carbide, silicon nitride, boronnitride and the like. Each of these may be used alone, or two or morekinds of these may be used in combination. Among the inorganic fineparticles, silicon carbide having superior thermal conductivity ispreferably used.

The sealing material layer 14 may be made from a compact material or maybe made from a porous material so as to allow exhaust gases to flowtherein; however, the sealing material layer 13 is preferably made froma compact material. This is because the sealing material layer 13 isformed so as to prevent leak of exhaust gases from the periphery of theceramic block 15 when the honeycomb structural body 10 of the presentinvention is placed in an exhaust passage of an internal combustionengine.

FIG. 3(a) is a perspective view that schematically shows a specificexample of an integral honeycomb structural body that is one example ofa honeycomb structural body of the present invention, and FIG. 3(b) is across-sectional view taken along line B-B of FIG. 3(a). Here, in thehoneycomb structural body shown in FIG. 3, a large number of throughholes are constituted by two kinds of through holes, that is,large-capacity through holes each of which has an area on a crosssection perpendicular to the length direction that is relativelygreater, and small-capacity through holes each of which has an area onthe cross section that is relatively smaller.

As shown in FIG. 3(a), the honeycomb structural body 30 includes acolumnar porous ceramic block 35 in which a large number of throughholes 31 are placed in parallel with one another in the length directionwith a partition wall 33 interposed therebetween. The through holes 31are constituted by two kinds of through holes, that is, large-capacitythrough holes 31 a each of which has an area on a cross sectionperpendicular to the length direction that is relatively greater, andsmall-capacity through holes 31 b each of which has an area on the crosssection perpendicular to the length direction that is relativelysmaller, and each of the large-capacity through holes 31 a is sealedwith a plug 32 at an end on the exhaust-gas outlet side of the honeycombstructural body 30, while each of the small-capacity through holes 31 bis sealed with a plug 32 at an end on the exhaust-gas inlet side of thehoneycomb structural body 30, so that a partition wall 33 that separatethe through holes 31 is allowed to serve as filters.

Although not shown in FIG. 3, a sealing material layer may be formed onthe circumference of the porous ceramic block 35 in the same manner asthe honeycomb structural body 10 shown in FIG. 1.

Except that the porous ceramic block 35 has an integral structure formedthrough a sintering process, the honeycomb structural body 30 has thesame structure as the aggregated honeycomb structural body 10 so thatexhaust gases that have entered the large-capacity through holes 31 aare allowed to flow out of the small-capacity through holes 31 b afterpassing through the partition wall 33 that separates the through holes31. Therefore, the integral honeycomb structural body 30 also has thesame effects as those of the aggregated honeycomb structural body.

In the same manner as the aggregated honeycomb structural body 10, theshape and size of the integral honeycomb structural body 30 may also bedetermined desirably, and the porosity thereof is preferably set in arange from 20 to 80%, with the pore diameter being preferably set in arange from 1 to 100 μm, in the same manner as the aggregated honeycombstructural body.

With respect to the porous ceramics constituting the porous ceramicblock 35, not particularly limited, the same nitride, carbide and oxideceramics used in the aggregated honeycomb structural body may beproposed, and in general, oxide ceramics such as cordierite are used.These materials make it possible to cut manufacturing costs, and sincethese materials have a comparatively small coefficient of thermalexpansion, it is possible to make the honeycomb structural body lesssusceptible to damage due to a thermal stress that is exerted duringproduction as well as during use.

The plug 32 to be used in the integral honeycomb structural body 30 isalso preferably made from porous ceramics, and with respect to thematerial thereof, although not particularly limited, for example, thesame materials as the ceramic materials used for forming theabove-mentioned porous ceramic block 35 may be used.

In the honeycomb structural body of the present invention having thestructures as shown in FIGS. 1 and 3, the density of the through holeson the cross section perpendicular to the length direction is preferablyset in a range from 15.5 to 62 (pcs/cm²)

When the density of the through holes on the cross section perpendicularto the length direction exceeds 62 (pcs/cm²), the cross-sectional areaof each of the through holes becomes too small, with the result thatashes tend to form bridges to cause clogging; in contrast, in the casewhen the density of the through holes is less than 15.5 (pcs/cm²), sincethe filtering area reduces in the honeycomb structural body as a whole,the pressure loss, caused upon collecting particulates, becomes greater,resulting in a great load on the engine and the subsequent instabilityin the discharge amount of particulates. Consequently, the collectingstate of particulates also becomes unstable, with the result that uponregenerating, ashes tend to form bridges to cause clogging in the poresand the subsequent increase in the pressure loss.

Moreover, in the above-mentioned honeycomb structural body, the shape ofa cross-section perpendicular to the length direction of each of thosethrough holes (large-capacity through holes and/or small-capacitythrough holes) is preferably formed into a polygonal shape, morepreferably, a quadrangle or an octagon.

This polygonal shape eliminates portions of the through hole that causegreater friction when exhaust gases are allowed to pass through thelarge-capacity through hole and/or the small-capacity through hole dueto the shape of the through hole, and consequently reduces a pressureloss caused by the friction of exhaust gases upon passing through thethrough hole, and also eliminates portions of a partition wall withirregular thicknesses, that is, portions that locally make it difficultfor exhaust gases to pass through so as to reduce a pressure loss causedby resistance of a partition wall exerted when exhaust gases passthrough the partition wall; thus, the polygonal shape is allowed toexert either of the above-mentioned effects.

Moreover, among polygonal shapes, a polygonal shape of a quadrangle ormore is preferably used, and at least one of the corners is preferablyformed as an obtuse angle. With this arrangement, it becomes possible toreduce a pressure loss caused by friction of exhaust gases upon flowingthrough the through hole inlet side or friction of exhaust gases uponflowing through the through hole outlet side.

Furthermore, on the cross section perpendicular to the length direction,at least one angle at which a wall portion, shared by one large-capacitythrough hole and an adjacent large-capacity through hole, and a wallportion shared by one large-capacity through hole and an adjacentsmall-capacity through hole, are caused to intersect with each other ispreferably set to an obtuse angle.

The vicinity of each of corners on the cross section of thelarge-capacity through hole and/or the small-capacity through hole ispreferably formed by a curved line. By forming the corner into a curvedline, it becomes possible to prevent occurrence of cracks caused by astress concentration at the corner.

In the present invention, the ratio of areas (the group oflarge-capacity through holes/the group of small-capacity through holes)on the cross section between the group of large-capacity through holesand the group of small-capacity through holes is preferably set in arange from 1.01 to 6. When the ratio of areas (the group oflarge-capacity through holes/the group of small-capacity through holes)exceeds 6, the capacity of the group of small-capacity through holesbecomes too small, with the result that the pressure loss, caused byfriction upon passing through the through-hole outlet side andresistance upon passing through the partition wall, increases to causean increase in the initial pressure loss. The ratio of the areas (thegroup of large-capacity through holes/the group of small-capacitythrough holes) is preferably set in a range from 1.2 to 5. Morepreferably, the ratio of the areas (the group of large-capacity throughholes/the group of small-capacity through holes) is set in a range from1.2 to 3.0.

Moreover, the ratio of areas (the group of large-capacity throughholes/the group of small-capacity through holes) on the cross sectionbetween the group of large-capacity through holes and the group ofsmall-capacity through holes is preferably set in a range from 1.01 to6. The ratio of areas (the group of large-capacity through holes/thegroup of small-capacity through holes) is also referred to as anaperture ratio. When the aperture ratio exceeds 6, the capacity of thegroup of small-capacity through holes becomes too small, with the resultthat the pressure loss, caused by friction upon passing through thethrough-hole outlet side and resistance upon passing through thepartition wall, increases to cause an increase in the initial pressureloss. The above-mentioned aperture ratio is preferably set in a rangefrom 1.2 to 5. Moreover, the above-mentioned aperture ratio is morepreferably set in a range from 1.2 to 3.0.

FIGS. 4(a) to 4(d) as well as FIGS. 5(a) to 5(f) are cross-sectionalviews each of which schematically shows one portion of the cross sectionof a porous ceramic member constituting the aggregated honeycombstructural body in accordance with the present invention, and FIG. 6 isa cross-sectional view that schematically shows a cross section of aporous ceramic member constituting the integral honeycomb structuralbody in accordance with the present invention. Here, regardless of theintegral type and the aggregated type, the shapes of the cross sectionsof the large-capacity through hole and the small-capacity through holeare respectively the same; therefore, referring to these Figures, thecross-sectional shapes of the large-capacity through hole and thesmall-capacity through hole in the honeycomb structural body of thepresent invention are explained.

In FIG. 4(a), the aperture ratio is almost 1.55, in FIG. 4(b), it isalmost 2.54, in FIG. 4(c), it is almost 4.45 and in FIG. 4(d), it isalmost 6.00. Moreover, in FIGS. 5(a), 5(c) and 5(e), all the apertureratios are almost 4.45, in FIGS. 5(b), 5(d) and 5(f), all the apertureratios are almost 6.0, and in FIG. 6, the aperture ratio is 3.0.

In FIGS. 4(a) to 4(d), each of the cross-sectional shapes of thelarge-capacity through holes is an octagon, and each of thecross-sectional shapes of the small-capacity through holes is aquadrangle (square), and these are alternately arranged; thus, bychanging the cross-sectional area of each of the small-capacity throughholes, with the cross-sectional shape of each of the large-capacitythrough holes being slightly changed, it is possible to desirably changethe aperture ratio easily. In the same manner, with respect to thehoneycomb filters shown in FIGS. 5 and 6, the aperture ratios thereofcan be desirably changed.

Here, in honeycomb structural bodies 160 and 260 shown in FIGS. 5(a) and5(b), each of the cross-sectional shapes of the large-capacity throughholes 161 a and 261 a is a pentagon with three corners thereof being setto almost right angles, and each of the cross-sectional shapes of thesmall-capacity through holes 161 b and 261 b is a quadrangle, and therespective quadrangles are placed at portions of a greater quadrangle,which diagonally face each other. Honeycomb structural bodies 170 and270, shown in FIGS. 5(c) and 5(d), have modified shapes of the crosssections shown in FIGS. 4(a) to 4(d) so that each partition wall sharedby each of the large-capacity through holes 171 a, 271 a and each of thesmall-capacity through holes 171 b, 271 b is expanded toward thesmall-capacity through hole side with a certain curvature. Thiscurvature may be arbitrarily set.

In this case, the curved line, which constitutes the partition wallshared by each of the large-capacity through holes 171 a, 271 a and eachof the small-capacity through holes 171 b, 271 b, corresponds to ¼ of acircle.

In honeycomb structural bodies 180 and 280 shown in FIGS. 5(e) to 5(f),the large-capacity through holes 181 a, 281 a and the small-capacitythrough holes 281 b, 281 b are formed into quadrangles (rectangularshapes), and as shown in Figures, these through holes are arranged sothat, when the two large-capacity through holes and the twosmall-capacity through holes are combined with one another, an almostsquare shape is formed.

In a honeycomb structural body 60 shown in FIG. 6, a square-shapedsmall-capacity through hole 61 b is formed at each of portionscorresponding to crossing points of a rectangular arrangement, and eachlarge-capacity through hole 61 a has a square shape with four cornersbeing chipped with small quadrangles, and a partition wall 62 a and 62 bseparating these are formed.

In the present invention, the distance between centers of gravity ofcross sections perpendicular to the length direction of adjacentlarge-capacity through holes is preferably designed to be equal to thedistance between centers of gravity of cross sections perpendicular tothe length direction of adjacent small-capacity through holes.

The term “the distance between centers of gravity of the cross sectionsof adjacent large-capacity through holes” represents a smallest distancebetween the center of gravity on a cross section perpendicular to thelength direction of one large-capacity through hole and the center ofgravity on a cross section perpendicular to the length direction of anadjacent large-capacity through hole; and the term “the distance betweencenters of gravity of the cross sections of adjacent small-capacitythrough holes” represents a smallest distance between the center ofgravity on a cross section perpendicular to the length direction of onesmall-capacity through hole and the center of gravity on a cross sectionperpendicular to the length direction of an adjacent small-capacitythrough hole.

In the case when the above-mentioned two distances between centers ofgravity are equal to each other, since heat is uniformly dispersed uponregenerating, it is possible to prevent the temperature inside thehoneycomb structural body from being locally distributed in a biasedmanner, and consequently to provide a filter having superior durabilityfree from cracks caused by a thermal stress, even after a long-term use.

When the honeycomb structural body of the present invention is used as afilter, collected particulates are gradually deposited on the inside ofeach of the through holes constituting the honeycomb structural body.

In the present invention, since the surface roughness (greatest height)R_(y) of the wall face of the throughhole, measured based upon JIS B0601, is set in a range from 10 to 100 μm, the pores and grains on thethrough hole wall face are properly placed to form appropriateirregularities so that particulates are allowed to deposit on the wallface of each through hole uniformly because of such appropriateirregularities; thus, it becomes possible to suppress the pressure lossupon collecting particulates to a low level.

Moreover, as the amount of deposited particulates becomes greater, thepressure loss increases gradually, and when it exceeds a predeterminedvalue, the load imposed on the engine becomes too high; therefore, thefilter is regenerated by burning the particulates.

In addition to carbon and the like that are burned to disappear, theparticulates include metals and the like that form oxides when burned,with the result that even after the particulates have been burned, theoxides and the like of these metals remain in the filter as ashes.

The way how the ashes remain is greatly influenced by the filterstructure and the like; however, in the present invention, since thesurface roughness (greatest height) R_(y) of the wall face of thethrough hole, measured based upon JIS B 0601, is set in a range from 10to 100 μm, as described above, the ashes are easily moved to theexhaust-gas outlet side through the through holes upon carrying out theregenerating process so that the wall face of each through hole becomesless susceptible to clogging; therefore, the capacity of eachlarge-capacity through hole is effectively utilized, the pressure lossis maintained in a low level for a long time to reduce the load imposedon the engine, and it becomes possible to provide a honeycomb structuralbody having a long service life. Consequently, it becomes possible tocut maintenance costs required for back washing and the like.

The following description will discuss one example of a manufacturingmethod for the honeycomb structural body of the present invention. Inthe case when the structure of the honeycomb structural body of thepresent invention is prepared as an integral honeycomb structural bodyconstituted by one sintered body as a whole as shown in FIG. 3, first,an extrusion molding process is carried out by using the above-mentionedmaterial paste mainly composed of ceramics to manufacture a ceramicformed body having almost the same shape as the honeycomb structuralbody 30 shown in FIG. 3.

In this case, metal molds to be used for extrusion-molding two types ofthrough holes, that is, for example, the large-capacity through holesand the small-capacity through holes, are properly selected inassociation with the density of each of the through holes.

With respect to the material paste, not particularly limited as long asthe porosity of the porous ceramic block that has been manufactured isset in a range from 20 to 80%, for example, the aforementioned material,prepared by adding a binder and a dispersant solution to powder madefrom ceramics, may be used.

With respect to the above-mentioned binder, not particularly limited,examples thereof include: methylcellulose, carboxy methylcellulose,hydroxy ethylcellulose, polyethylene glycol, phenolic resin, epoxy resinand the like.

In general, the blended amount of the above-mentioned binder ispreferably set to 1 to 10 parts by weight with respect to 100 parts byweight of ceramic powder.

With respect to the dispersant solution, not particularly limited,examples thereof include: an organic solvent such as benzene; alcoholsuch as methanol; water and the like.

An appropriate amount of the above-mentioned dispersant solution ismixed therein so that the viscosity of the material paste is set withina fixed range.

These ceramic powder, binder and dispersant solution are mixed by anattritor or the like, and sufficiently kneaded by a kneader or the like,and then extrusion-molded so that the above-mentioned ceramic formedbody is manufactured.

Moreover, a molding auxiliary may be added to the material paste, ifnecessary.

With respect to the molding auxiliary, not particularly limited,examples thereof include: ethylene glycol, dextrin, fatty acid soap,polyalcohol and the like.

Furthermore, a pore-forming agent, such as balloons that are fine hollowspheres composed of oxide-based ceramics, spherical acrylic particlesand graphite, may be added to the above-mentioned material paste, ifnecessary.

With respect to the above-mentioned balloons, not particularly limited,for example, alumina balloons, glass micro-balloons, shirasu balloons,fly ash balloons (FA balloons) and mullite balloons may be used. Amongthese, fly ash balloons are more preferably used.

In the honeycomb structural body of the present invention, the surfaceroughness R_(y) of the wall face of the through hole, measured basedupon JIS B 0601, is set in a range from 10 to 100 μm; therefore, inorder to allow the through-hole wall face of the produced honeycombstructural body to have a roughened surface having the above-mentionedroughness, the through-hole surface forming portion of a metal mold tobe used for the extrusion molding process is roughened through anappropriate method. The surface roughness can be changed by changingdrying conditions in the following drying process; however, since crackstend to occur in the raw molded body depending on conditions,preferably, drying conditions are not changed from those of theconventional manufacturing method.

Moreover, by changing the density (porosity) of the honeycomb structuralbody, the surface roughness of the through hole can be changed. In thiscase, by changing the combination of particle sizes of two kinds ofceramic powders contained in the material paste, the density of thehoneycomb structural body can be changed.

Next, after the above-mentioned ceramic formed body has been dried byusing a drier such as a microwave drier, a hot-air drier, a dielectricdrier, a reduced-pressure drier, a vacuum drier and a frozen drier,predetermined through holes are filled with plug paste to form plugs sothat a mouth-sealing process for plugging the through holes is carriedout.

With respect to the above-mentioned plug paste, not particularly limitedas long as the porosity of a plug manufactured through post-processes isset in a range from 20 to 80%, for example, the same material paste asdescribed above may be used; however, those pastes, prepared by adding alubricant, a solvent, a dispersant and a binder to ceramic powder usedas the above-mentioned material paste, are preferably used. With thisarrangement, it becomes possible to prevent ceramics particles in theplug paste from settling in the middle of the sealing process.

Next, the ceramic dried body filled with the plug paste is subjected todegreasing and sintering processes under predetermined conditions sothat a honeycomb structural body constituted by a single sintered bodyas a whole is manufactured.

Here, with respect to the degreasing and sintering conditions and thelike of the ceramic dried body, it is possible to apply conditions thathave been conventionally used for manufacturing a honeycomb structuralbody made from porous ceramics.

The roughness of the through-hole wall face may be adjusted bysubjecting the through holes of the resulting honeycomb structural bodyto a roughening process such as a sand blasting process.

In the case when the structure of the honeycomb structural body of thepresent invention is prepared as an aggregated honeycomb structural bodyconstituted by a plurality of porous ceramic members combined with oneanother through sealing material layers as shown in FIG. 1, first, anextrusion molding process is carried out by using the above-mentionedmaterial paste mainly composed of ceramics to manufacture a raw ceramicformed body having a shape like a porous ceramic member 20 shown in FIG.2. At this time, in order to allow the through-hole wall face of theproduced honeycomb structural body to have a roughened surface havingpredetermined roughness, the through-hole surface forming portion of ametal mold to be used for the extrusion molding process is roughenedthrough an appropriate method.

Here, with respect to the material paste, the same material paste asexplained in the above-mentioned aggregated honeycomb structural bodymay be used.

After the above-mentioned raw molded body has been dried by using amicrowave drier or the like to form a dried body, plug paste, whichforms plugs, is injected into predetermined through holes of the driedbody so that sealing processes for sealing the through holes are carriedout.

Here, with respect to the plug paste, the same plug paste as thatexplained in the above-mentioned integral honeycomb structural body maybe used, and with respect to the sealing process, the same method as themethod for the above-mentioned integral honeycomb structural body may beused except that the subject to be filled with the plug paste isdifferent.

Next, the dried body that has been subjected to the sealing process issubjected to degreasing and sintering processes under predeterminedconditions so that a porous ceramic member in which a plurality ofthrough holes are placed in parallel with one another in the lengthdirection with a partition wall interposed therebetween is manufactured.

Here, with respect to the conditions and the like of degreasing andsintering processes for the raw molded body, those conditionsconventionally used for manufacturing a honeycomb structural bodyconstituted by a plurality of porous ceramic members that are combinedwith one another through sealing material layers may be used.

Next, sealing material paste to be used for forming a sealing materiallayer 14 is applied with an even thickness to form a sealing materialpaste layer, and on this sealing material paste layer, a process forlaminating another porous ceramic member 20 is successively repeated sothat a laminated body of porous ceramic members 20 having a rectangularpillar shape with a predetermined size is manufactured.

With respect to the material for forming the sealing material paste,since the same material as that explained in the honeycomb structuralbody of the present invention can be used, the description thereof isomitted.

Next, the laminated body of the porous ceramic member 20 is heated sothat the sealing material paste layer is dried and solidified to formthe sealing material layer 14; thereafter, by cutting the peripheralportion into, for example, a shape as shown in FIG. 1, by using adiamond cutter or the like so that a ceramic block 15 is manufactured.

A sealing material layer 13 is formed on the circumference of theceramic block 15 by using the sealing material paste so that a honeycombstructural body in which a plurality of porous ceramic members arecombined with one another through sealing material layers ismanufactured.

Any of the honeycomb structural bodies thus produced have a pillarshape, and the structures thereof are shown in FIGS. 1 and 2.

With respect to the application of the honeycomb structural body of thepresent invention, although not particularly limited, it is preferablyused for exhaust gas purifying devices for use in vehicles.

FIG. 7 is a cross-sectional view that schematically shows one example ofan exhaust gas purifying device for use in vehicles, which is providedwith the honeycomb structural body of the present invention.

As shown in FIG. 7, an exhaust gas purifying device 800 is mainlyconstituted by a honeycomb structural body 80 of the present invention,a casing 830 that covers the external portion of the honeycombstructural body 80, a holding sealing material 820 that is placedbetween the honeycomb structural body 80 and the casing 830 and aheating means 810 placed on the exhaust-gas inlet side of the honeycombstructural body 80, and an introducing pipe 840, which is connected toan internal combustion device such as an engine, is connected to one endof the casing 830 on the exhaust gas inlet side, and an exhaust pipe 850externally coupled is connected to the other end of the casing 830. InFIG. 7, arrows show flows of exhaust gases.

Moreover, in FIG. 7, the honeycomb structural body 80 may be prepared asthe honeycomb structural body 10 shown in FIG. 1 or as the honeycombstructural body 30 shown in FIG. 3.

In the exhaust gas purifying device 800 having the above-mentionedarrangement, exhaust gases, discharged from the internal-combustionsystem such as an engine, are directed into the casing 830 through theintroducing pipe 840, and allowed to flow into the honeycomb structuralbody 80 through the inlet side through holes and to pass through thewall portion (a partition wall); thus, the exhaust gases are purified,with particulates thereof being collected in the wall portion (apartition wall), and are then discharged outside through the exhaustpipe 850.

After a large quantity of particulates have been accumulated on the wallportion (the partition wall) of the honeycomb structural body 80 tocause an increase in pressure loss, the honeycomb structural body 80 issubjected to a regenerating process.

In the regenerating process, a gas, heated by using a heating means 810,is allowed to flow into the through holes of the honeycomb structuralbody 80 so that the honeycomb structural body 80 is heated to burn andeliminate the particulates deposited on the wall portion (partitionwall)

Moreover, in the present invention, in addition to the above-mentionedmethod, the particulates may be burned and eliminated by using apost-injection system.

Moreover, the honeycomb structural body of the present invention mayhave a catalyst capable of purifying CO, HC, NOx and the like in theexhaust gases.

When such a catalyst is supported thereon, the honeycomb structural bodyof the present invention is allowed to function as a honeycombstructural body capable of collecting particulates in exhaust gases, andalso to function as a catalyst converter for purifying CO, HC, NOx andthe like contained in exhaust gases. Moreover, depending on cases, thehoneycomb structural body makes it possible to lower the burningtemperature of the particulates.

With respect to the catalyst, examples thereof include noble metals suchas platinum, palladium, rhodium and the like. The catalyst, made from anoble metal such as platinum, palladium, rhodium and the like, is aso-called three-way catalyst, and the honeycomb structural body of thepresent invention which is provided with such a three-way catalyst isallowed to function in the same manner as conventionally known catalystconverters. Therefore, with respect to the case in which the honeycombstructural body of the present invention also functions as a catalystconverter, detailed description thereof is omitted.

Here, with respect to the catalyst that is supported on the honeycombstructural body of the present invention, not particularly limited tothe above-mentioned noble metal, any catalyst may be supported as longas it can purify CO, HC, NOx and the like contained in exhaust gases.

EXAMPLES

The following description will discuss the present invention in detailby means of examples; however, the present invention is not intended tobe limited by these examples.

Example 1

(1) Powder of α-type silicon carbide having an average particle size of11 μm (60% by weight) and powder of β-type silicon carbide having anaverage particle size of 0.5 μm (40% by weight) were wet-mixed, and to100 parts by weight of the resulting mixture were added and kneaded 5parts by weight of an organic binder (methyl cellulose) and 10 parts byweight of water to obtain a mixed composition. Next, after a slightamount of a plasticizer and a lubricant have been added and kneadedtherein, the resulting mixture was extrusion-molded by using a metalmold having a surface roughness Ra of 10 μm at the portionscorresponding to the through holes so that a raw molded product, whichhad almost the same cross-sectional shape as each of the cross-sectionalshapes shown in FIGS. 4(a) to 4(d), was manufactured with an apertureration of 2.54.

Next, the above-mentioned raw molded product was dried by using amicro-wave drier to form a ceramics dried body, and after predeterminedthrough holes had been filled with a plug paste having the samecomposition as the molded product, the resulting product was again driedby using a drier, and then degreased at 400° C., and sintered at 2200°C. in a normal-pressure argon atmosphere for 3 hours to manufacture aporous ceramic member 20, which was a silicon carbide sintered body, andhad a porosity of 42%, an average pore diameter of 9 μm, a size of 34.3mm×34.3 mm×150 mm, the number of through holes of 23.3/cm² and athickness of almost all the partition wall 23 of 0.41 mm, withlarge-capacity through holes and small-capacity through holes.

Here, on one end face of the columnar porous ceramic member 20, only thelarge-capacity through holes 21 a were sealed with plugs, and on theother end face thereof, only the small-capacity through holes 21 b weresealed with plugs.

(2) By using a heat resistant sealing material paste containing 30% byweight of alumina fibers having a fiber length of 0.2 mm, 21% by weightof silicon carbide particles having an average particle size of 0.6 μm,15% by weight of silica sol, 5.6% by weight of carboxymethyl celluloseand 28.4% by weight of water, a large number of the porous siliconcarbide members were combined with one another, and this was then cut byusing a diamond cutter to form a cylindrical shaped ceramic block.

In this case, the thickness of the sealing material layers used forcombining the porous ceramic members was adjusted to 1.0 mm.

Next, ceramic fibers made from alumina silicate (shot content: 3%, fiberlength: 0.1 to 100 mm) (23.3% by weight), which served as inorganicfibers, silicon carbide powder having an average particle size of 0.3 μm(30.2% by weight), which served as inorganic particles, silica sol (SiO₂content in the sol: 30% by weight) (7% by weight), which served as aninorganic binder, carboxymethyl cellulose (0.5% by weight), which servedas an organic binder, and water (39% by weight) were mixed and kneadedto prepare a sealing material paste.

Next, a sealing material paste layer having a thickness of 0.2 mm wasformed on the circumferential portion of the ceramic block by using theabove-mentioned sealing material paste. Further, this sealing materialpaste layer was dried at 120° C. so that a cylinder-shaped honeycombstructural body having a diameter of 144 mm was produced.

The surface roughness of the wall faces constituting the through holesof the resulting honeycomb structural body and the porosity of thehoneycomb structural body are shown in Table

Examples 2 to 6 and 11 to 13)

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1, with the wall thickness being set to a value shown inTable 1, so that a porous ceramic member was manufactured, and ahoneycomb structural body was then produced. The wall thickness, thesurface roughness R_(y) of through-hole wall faces, the density of thethrough holes and the porosity of the resulting honeycomb structuralbody are shown in Table 1.

Example 7

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1, with the sintering conditions changed to 2000° C. and3 hours, so that a porous ceramic member was manufactured, and ahoneycomb structural body was then produced.

The wall thickness, the surface roughness R_(y) of through-hole wallfaces, the density of the through holes and the porosity of theresulting honeycomb structural body are shown in Table 1.

Example 8

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1, with the sintering conditions changed to 2200° C. and1 hour, so that a porous ceramic member was manufactured, and ahoneycomb structural body was then produced.

The wall thickness, the surface roughness R_(y) of through-hole wallfaces, the density of the through holes and the porosity of theresulting honeycomb structural body are shown in Table 1.

Example 9

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1, and that a mixed composition was obtained by using 100parts by weight of a mixture made from 80% by weight of powder of α-typesilicon carbide having an average particle size of 50 μm and 20% byweight of powder of β-type silicon carbide having an average particlesize of 0.5 μn, 15 parts by weight of an organic binder (methylcellulose) and 20 parts by weight of water, with the sinteringconditions changed to 2300° C. and 3 hours, so that a porous ceramicmember was manufactured, and a honeycomb structural body was thenproduced.

The wall thickness, the surface roughness R_(y) of through-hole wallfaces, the density of the through holes and the porosity of theresulting honeycomb structural body are shown in Table 1.

Example 10

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1, and that a mixed composition was obtained by using 100parts by weight of a mixture made from 80% by weight of powder of α-typesilicon carbide having an average particle size of 50 μm and 20% byweight of powder of β-type silicon carbide having an average particlesize of 0.5 μm, 15 parts by weight of an organic binder (methylcellulose) and 20 parts by weight of water, with the sinteringconditions changed to 2300° C. and 6 hours, so that a porous ceramicmember was manufactured, and a honeycomb structural body was thenproduced.

The wall thickness, the surface roughness R_(y) of through-hole wallfaces, the density of the through holes and the porosity of theresulting honeycomb structural body are shown in Table 1.

Comparative Example 1

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1 and that the wall thickness was set to a value shown inTable 1 so that a porous ceramic member was manufactured, and ahoneycomb structural body was then produced. The wall thickness, thesurface roughness R_(y) of through-hole wall faces, the density of thethrough holes and the porosity of the resulting honeycomb structuralbody are shown in Table 1.

Here, the honeycomb structural body according to Comparative Example 1corresponds to a honeycomb structural body 400 shown in FIG. 8, and thecross-sectional area of each of through holes 401 that are formedbetween wall portions 402 has the same value except for the endportions.

Comparative Example 2

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1, and that a mixed composition was obtained by using 100parts by weight of a mixture made from 60% by weight of powder of α-typesilicon carbide having an average particle size of 11 μm and 40% byweight of powder of β-type silicon carbide having an average particlesize of 0.5 μm, 5 parts by weight of an organic binder (methylcellulose) and 20 parts by weight of water, with the sinteringconditions changed to 1800° C. and 3 hours, so that a porous ceramicmember was manufactured, and a honeycomb structural body was thenproduced.

The wall thickness, the surface roughness R_(y) of through-hole wallfaces, the density of the through holes and the porosity of theresulting honeycomb structural body are shown in Table 1.

Comparative Example 3

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1, and that a mixed composition was obtained by using 100parts by weight of a mixture made from 80% by weight of powder of α-typesilicon carbide having an average particle size of 50 μm and 20% byweight of powder of β-type silicon carbide having an average particlesize of 0.5 μm, 15 parts by weight of an organic binder (methylcellulose) and 20 parts by weight of water, with the sinteringconditions changed to 2300° C. and 12 hours, so that a porous ceramicmember was manufactured, and a honeycomb structural body was thenproduced.

The wall thickness, the surface roughness R_(y) of through-hole wallfaces, the density of the through holes and the porosity of theresulting honeycomb structural body are shown in Table 1.

Comparative Example 4

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1, and that a mixed composition was obtained by using 100parts by weight of a mixture made from 80% by weight of powder of α-typesilicon carbide having an average particle size of 50 μm and 20% byweight of powder of β-type silicon carbide having an average particlesize of 0.5 μm, 15 parts by weight of an organic binder (methylcellulose) and 20 parts by weight of water, with the sinteringconditions changed to 2300° C. and 24 hours, so that a porous ceramicmember was manufactured, and a honeycomb structural body was thenproduced.

The wall thickness, the surface roughness R_(y) of through-hole wallfaces, the density of the through holes and the porosity of theresulting honeycomb structural body are shown in Table 1.

Reference Example 1

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in FIG. 9 so that a porous ceramic member was manufactured, and ahoneycomb structural body was then produced. The wall thickness, thesurface roughness R_(y) of through-hole wall faces, the density of thethrough holes and the porosity of the resulting honeycomb structuralbody are shown in Table 1.

Here, the honeycomb structural body according to Reference Example 1corresponds to a honeycomb structural body 200 shown in FIG. 9, and thethrough holes thereof are constituted by large-capacity through holes201 each of which has across section having a hexagonal shape andsmall-capacity through holes 202 each of which has a cross sectionhaving a triangular shape, with the number of the small-capacity throughholes 202 being set to about twice as many as the number of thelarge-capacity through holes 201.

Reference Examples 2 and 3

The same processes as Example 1 were carried out except that in theprocess (1), the cross-sectional shapes of the large-capacity throughholes and the small-capacity through holes were formed into shapes asshown in Table 1 and that the wall thickness was set to a value shown inTable 1 so that a porous ceramic member was manufactured, and ahoneycomb structural body was then produced. The wall thickness, thesurface roughness R_(y) of through-hole wall faces, the density of thethrough holes and the porosity of the resulting honeycomb structuralbody are shown in Table 1.

(Evaluation Method)

(1) Surface Roughness Measurements on Through-Hole Wall Face

Each of the honeycomb structural bodies according to the examples,comparative examples and reference examples was cut in parallel with thethrough hole so that the through hole is exposed, and the surfaceroughness of the through hole was measured by using a surface roughnessmeasuring device (SURFCOM 920A, made by Tokyo Seimitsu Co., Ltd.) andbased upon the results, the surface roughness R_(y) was determined incompliance with JIS B 0601. Table 1 shows the results.

(2) Pressure Loss Variations

As shown in FIG. 7, each of the honeycomb structural bodies of theexamples, comparative examples and reference examples was placed in anexhaust passage of an engine to form an exhaust gas purifying device,and the engine was driven at the number of revolutions of 3000 min⁻¹ anda torque of 50 Nm for a predetermined period of time so that the amountof collected particulates was measured and the pressure loss wasmeasured. The value of the initial pressure loss at this time and thepressure loss at the amount of collected particulates of 6 (g/L) areshown in Table 1.

(3) Relationship Between the Weight of Ashes and Pressure Loss

As shown in FIG. 7, each of the honeycomb structural bodies of theexamples, comparative examples and reference examples was placed in anexhaust passage of an engine to form an exhaust gas purifying device,and the engine was driven at the number of revolutions of 3000 min⁻¹ anda torque of 50 Nm for a predetermined period of time; thereafter,experiments for repeating regenerating processes were carried out sothat the weight of ashes accumulated in the through holes constitutingthe honeycomb structural body was measured and the pressure loss of thehoneycomb structural body was measured. The value of the pressure lossat the time of 150 g of accumulated ashes is shown in Table 1.

FIG. 10 is a graph that indicates a relationship between thethrough-hole density and the pressure loss, and FIG. 11 is a graph thatindicates a relationship between the surface roughness of thethrough-hole wall face and the pressure loss.

(4) Measurements on Porosity

The porosity was measured by using Archimedes method. The results areshown in Table 1. TABLE 1 Surface Pressure loss roughness upon Pressureof through Initial collecting 6 loss upon Wall hole wall Density ofpressure (g/L) of depositing Sectional thickness face through holePorosity loss particulates 150 g of shape (mm) Ry (μm) (pcs/cm²) (vol %)(kPa) (kPa) ashes (kPa) Example 1 FIGS. 4(a)-4(d) 0.41 33 23.3 42 4.88.7 10.0 Example 2 FIGS. 5(c)-5(d) 0.41 33 23.3 42 4.9 8.8 10.4 Example3 FIG. 5(e) 0.41 33 23.3 42 4.9 8.6 10.2 Example 4 FIG. 5(f) 0.41 3323.3 42 4.9 8.6 10.2 Example 5 FIGS. 5(a)-5(b) 0.41 33 23.3 42 5.0 8.710.3 Example 6 0.41 33 23.3 42 5.9 9.9 10.8 Example 7 FIGS. 4(a)-4(d)0.41 15 23.3 42 6.3 10.5 14.6 Example 8 FIGS. 4(a)-4(d) 0.41 20 23.3 425.0 9.0 10.7 Example 9 FIGS. 4(a)-4(d) 0.41 70 23.3 50 4.4 11.5 11.9Example 10 FIGS. 4(a)-4(d) 0.41 90 23.3 50 4.4 11.7 13.2 Example 11FIGS. 4(a)-4(d) 0.41 33 15.5 42 4.2 8.9 14.8 Example 12 FIGS. 4(a)-4(d)0.35 33 46.5 42 5.6 8.4 11.4 Example 13 FIGS. 4(a)-4(d) 0.35 33 54.3 425.8 8.2 13.8 Comparative 0.41 33 23.3 42 4.6 9.1 19.3 Example 1Comparative FIGS. 4(a)-4(d) 0.41 9 23.3 42 6.9 12.0 22.5 Example 2Comparative FIGS. 4(a)-4(d) 0.41 110 23.3 50 4.0 12.0 23.0 Example 3Comparative FIGS. 4(a)-4(d) 0.41 120 23.3 55 3.9 12.1 23.3 Example 4Reference 0.41 33 23.3 42 6.7 10.7 18.6 Example 1 Reference FIGS.4(a)-4(d) 0.41 33 14.0 42 4.0 9.0 18.5 Example 2 Reference FIGS.4(a)-4(d) 0.35 33 69.8 42 6.5 9.2 18.7 Example 3

As clearly indicated by the results shown in Table 1 and FIGS. 10 and11, although there is no significant difference in the initial pressureloss in comparison with the honeycomb structural bodies according tocomparative examples, the honeycomb structural bodies of examples had asmaller increase in the pressure loss upon collection of 6 (g/L) ofparticulates as well as upon deposition of 150 g of ashes, when thedensity of the through holes was out of the range of the presentinvention as well as when the surface roughness of wall faces formingthe through holes was out of the range of the present invention. In thismanner, the present invention makes it possible to maintain the pressureloss upon collection of particulates at a low level and also to maintainthe pressure loss caused by deposition of ashes at a low level for along time; thus, it becomes possible to effectively utilize the capacityof the large-capacity through holes, to reduce the load imposed on anengine, and consequently to provide a honeycomb structural body having along service life. Consequently, it becomes possible to cut maintenancecosts required for back washing and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] a perspective view that schematically shows one example of ahoneycomb structural body of the present invention.

[FIG. 2] (a) is a perspective view that schematically shows one exampleof a porous ceramic member that constitutes the honeycomb structuralbody shown in FIG. 1; and (b) is a cross-sectional view taken along lineA-A of the porous ceramic member shown in (a).

[FIG. 3] (a) is a perspective view that schematically shows anotherexample of the honeycomb structural body of the present invention; and(b) is a cross-sectional view taken along line B-B of the honeycombstructural body shown in (a).

[FIGS. 4](a) to (d) are cross-sectional views each of whichschematically shows a cross section perpendicular to the lengthdirection of the porous ceramic member constituting the honeycombstructural body of the present invention.

[FIGS. 5] (a) to (f) are longitudinal cross-sectional views thatschematically show one example of the honeycomb structural body of thepresent invention.

[FIG. 6] a longitudinal cross-sectional view that schematically showsanother example of the honeycomb structural body of the presentinvention.

[FIG. 7] a cross-sectional view that schematically shows one example ofan exhaust gas purifying device using the honeycomb structural body ofthe present invention.

[FIG. 8] a cross-sectional view that schematically shows one example ofa conventional honeycomb structural body.

[FIG. 9] a cross-sectional view that schematically shows one example ofthe honeycomb structural body.

[FIG. 10] a graph that shows a relationship between the through-holedensity and the pressure loss of honeycomb structural bodies accordingto examples, comparative examples and reference examples.

[FIG. 11] a graph that shows a relationship between the surfaceroughness of through hole wall face and the pressure loss of thehoneycomb structural bodies according to the examples, the comparativeexamples and the reference examples.

EXPLANATION OF SYMBOLS

-   10, 30 honeycomb structural body-   13, 14 sealing material layer-   15 ceramic block-   20, 40, 50, 70 porous ceramic member-   21 a, 31 a, 41 a, 51 a, 71 a large-capacity through hole-   21 b, 31 b, 41 b, 51 b, 71 b small-capacity through hole-   22 plug-   23, 43, 53, 73 partition wall-   33 wall portion-   160, 170, 180, 260, 270, 280 porous ceramic member-   161 a, 171 a, 181 a, 261 a, 271 a, 281 a large-capacity through hole-   161 b, 171 b, 181 b, 261 b, 271 b, 281 b small-capacity through hole-   163, 173, 183, 263, 273, 283 wall portion-   60 porous ceramic member-   61 a large-capacity through hole-   61 b small-capacity through hole-   62 a, 62 b wall portion

1. A honeycomb structural body made of a columnar porous ceramic blockin which a large number of through holes are placed in parallel with oneanother in the length direction with a wall portion interposedtherebetween, wherein said large number of through holes comprises: agroup of large-capacity through holes, each of which is sealed at oneend of said honeycomb structural body such that the total sum of theareas thereof on a cross section perpendicular to the length directionis made relatively great; and a group of small-capacity through holes,each of which is sealed at the other end of said honeycomb structuralbody such that the total sum of the areas on said cross section is maderelatively small, a surface roughness R_(y) of the wall face of saidthrough hole being set in a range from 10 to 100 μm.
 2. The honeycombstructural body according to claim 1, wherein a density of the throughholes on a cross section perpendicular to the length direction is set ina range from 15.5 to 62 pcs/cm².
 3. The honeycomb structural bodyaccording to claim 1 or 2, wherein the large number of through holes areconstituted by two kinds of through holes, that is, large-capacitythrough holes each of which has a relatively greater area on a crosssection perpendicular to the length direction and small-capacity throughholes each of which has a relatively smaller area on said cross sectionperpendicular to the length direction.
 4. The honeycomb structural bodyaccording to any one of claims 1 to 3, wherein the shape of a crosssection perpendicular to the length direction of each of the throughholes is a polygonal shape.
 5. The honeycomb structural body accordingto any one of claims 1 to 4, wherein the shape of a cross sectionperpendicular to the length direction of each of the through holes is anoctagonal shape or a quadrangle shape.
 6. The honeycomb structural bodyaccording to any one of claims 1 to 5, wherein the ratio (group oflarge-capacity through holes/group of small-capacity through holes) ofareas of cross sections of the group of large-capacity through holes tothe group of small-capacity through holes is set in a range from 1.01 to6.
 7. The honeycomb structural body according to any one of claims 3 to6, wherein the ratio (large-capacity through holes/small-capacitythrough holes) of areas of cross sections of the large-capacity throughholes to the small-capacity through holes is set in a range from 1.01 to6.
 8. The honeycomb structural body according to any one of claims 3 to7, wherein on the cross section perpendicular to the length direction,at least one angle at which a part of a wall portion, shared by one ofsaid large-capacity through holes and an adjacent large-capacity throughhole, and a part of a wall portion, shared by one of said large-capacitythrough holes and an adjacent small-capacity through hole, are caused tointersect with each other is set to an obtuse angle.
 9. The honeycombstructural body according to any one of claims 3 to 8, wherein thevicinity of each of corners on the cross section of the large-capacitythrough hole and/or the small-capacity through hole is formed by acurved line.
 10. The honeycomb structural body according to any one ofclaims 3 to 9, wherein the distance between centers of gravity of crosssections perpendicular to the length direction of adjacently locatedlarge-capacity through holes is set to be equal to the distance betweencenters of gravity of cross sections perpendicular to the lengthdirection of adjacently located small-capacity through holes.
 11. Thehoneycomb structural body according to any one of claims 1 to 10,wherein the porous ceramic block is constituted by combining a pluralityof columnar porous ceramic members, each having a plurality of throughholes that are placed in parallel with one another in the lengthdirection with a partition wall interposed therebetween, with oneanother through sealing material layers.
 12. A filter for use in anexhaust gas purifying device used for a vehicle, wherein the honeycombstructural body according to any one of claims 1 to 11 is installed.